Boundary routers that advertise briefer information between areas, including info about subnets/masks, but info advertised into one area doesn’t include details about the topology of the other area.

What is a Link State Database?

The data structure held by an OSPF router for the purpose of storing topology data.

What is Shortest Path First (SPF)?

The name of the algorithm OSPF uses to analyze the LSDB. The analysis determines the best (lowest cost) route for each prefix/length.

What is Link State Update (LSU)?

The name of the OSPF packet that holds the detailed topology info, specifically LSAs.

What is a Link State Advertisement (LSA)?

The name of a class of OSPF data structures that hold topology info. LSAs are held in memory in the LSDB and communicated over the network in LSU messages.

What is an area?

A contiguous grouping of routers and router interfaces. Routers in an area strive to learn all topology info about the area, but they do not learn topology info about areas to which they do not connect.

What is an Area Border Router (ABR)?

A router that has interfaces connected to at least two different OSPF areas, including the backbone area. ABRs hold topology data for each area, and calculate routes for each area, and advertise about those routes between areas.

What is a Backbone router?

Any router that has at least one interface connected to the backbone area.

What is an Internal router?

A router that has interfaces connected to only one area, making the router completely internal to that one area.

What is a Designated Router (DR)?

On multiaccess data links like LANs, an OSPF router elected by the routers on that data link to perform special functions. These functions include the generation of LSAs representing the subnet, and playing a key role in the database exchange process.

What is a Backup Designated Router (BDR)?

A router on a multiaccess data link that monitors the DR and becomes prepared to take over for the DR, should the DR fail.

How is OSPF configured?

router ospf process-id

network net-id wildcard-mask area area-id

What are the rules for the OSPF configuration commands?

Neighboring routers’ router ospf process-id commands do not have to be the same process-id.

IOS only enables OSPF on interfaces matched by the OSPF network command. When enabled, the router does the following:

To match an interface w/ the network command, IOS compares the net-id configured w/ each interface’s IP address, while using the configured wildcard-mask as an ACL wildcard mask.

Regardless of the order in which the network commands are added to the config, IOS puts these commands into the config file w/ the most specific wildcard mask first. IOS lists the network commands in this sorted order in the configuration.

The first network command that matches an interface, per the order shown in the output of the show running-config command, determines the OSPF area number associated with the interface.

How does OSPF determine the router ID?

Use the router ID configured in the router-id x.x.x.x OSPF router subcommand.